1. Technical Field
The present disclosure relates to telecommunication and, more specifically, to wireless communication.
2. Description of Related Art
The new 4G wireless technology standard termed Long Term Evolution-Advanced (LTE-A) utilizes the well-known modulation scheme known as orthogonal frequency division multiple access (OFDMA). It is a multicarrier technique in which the transmit spectrum is divided into K orthogonal subcarriers equally spaced in frequency. The method has been used for many years in both wireline broadband communications and wireless local area networks (WLAN). LTE-A provides a minimum of 1000 Mbps throughput in the downlink (DL) and 500 Mbps in the uplink (UL). The spectral bandwidth for LTE-A is 100 MHz, using up to five component carriers each with a component bandwidth of up to 20 MHz. LTE-A also includes support for both frequency domain duplexing and time domain duplexing.
LTE-A also employs multiple antenna methods such as spatial multiplexing and transmit diversity. Spatial multiplexing (SM) is a multiple-input and multiple-output system (MIMO) formulation enabled by configuring multiple antennas separated in space. The spatially separated antennas provide separate and distinct transmission channels allowing the transmitter-receiver pair to extract independent signals from each channel while cancelling interference from the other transmission paths. When combined, OFDMA and MIMO-SM provide orthogonality in both frequency and space. LTE-A supports up to eight antennas per modem.
LTE is deployed in various cell-type structures with the coverage area varying from a macro-cell area of ˜20 km diameter to small-cells such as femtocells and microcells, which are much smaller with cell coverage areas sized for servicing buildings or small campus environments. These different cells may use the same frequency spectrum, which causes direct interference. Generally, LTE interference scenarios include the following: 1) interference between a small-cell and a macro-cell, and 2) interference between two macro-cells near the cell boundary.
Wireless communications systems need to contend with radio signal propagation impairments that include multipath fading, noise, and interference. Multipath fading results from a transmitted radio signal traversing many different paths from a transmitting antenna to a receiving antenna as a result of reflections from both man-made and natural environmental objects. The multiple reflected signals (including a possible line-of-sight signal) combine at the receiver to form a transmission path impulse response (with an associated transmission path frequency response). Depending on the characteristics of this response, it is possible for parts of the transmission channel to have deep nulls, which can be time-varying as a result of movement of the transmitter, the receiver, and the objects causing reflections. A wireless MIMO system has different transmission paths for its spatially separated antennas. MIMO signal processing increases the probability that the transmit signal can be correctly received at the decoder while some of the transmission paths are subject to harsh attenuation as a result of the multipath fading problem. Some examples of MIMO system configurations are shown in FIGS. 1A-1D. FIG. 1A illustrates a scenario of a single cell and a single unit (SU) having single-input and single-output (SISU) capability. FIG. 1B illustrates a scenario of a single cell and a single unit having multiple-input and single-output (MISO) capability. FIG. 1C illustrates a scenario of a single cell and a single unit having MIMO capability. FIG. 1D illustrates a scenario of multiple cells (two cells shown) and a single unit having MIMO capability.